1. Technical Field
The present disclosure relates to an ignition system for an internal combustion engine, the ignition system including a spark coil (ignition coil) that has a primary coil and a secondary coil electro-magnetically connected to the primary coil, and a spark plug that applies high voltage to a gap between a center electrode and a ground electrode on the basis of an electro-magnetic energy stored in the spark coil, thereby producing discharge sparks in between the electrodes.
2. Related Art
Recently as the trend of downsizing for vehicles progresses, a compression ratio in a spark-ignition internal-combustion engine (gasoline engine) tends to be increased by using a supercharger in order to improve fuel consumption and reduce costs. As the compression ratio become higher, an in-cylinder pressure (pressure in a cylinder) also becomes higher while discharge sparks are produced in the spark plug, thereby discharge voltage of the spark plug become higher. Once the discharge voltage becomes higher, at the time when the electrode of the spark plug is worn-out due to the increase of a running distance or the like, at an early stage from then, the discharge voltage may exceed an insulation-breakdown limit voltage of a plug insulator, thereby reliability of the spark plug is impaired. As a result, discharge sparks would no longer be produced and an accidental fire in the engine may occur.
As a measure against this, the inventors of the present disclosure have paid attention to a technique as disclosed in JP-B-H06-080313. The technique makes use of a constant-voltage element, such as a Zener diode or a Varistor, to restrict the discharge voltage of a spark plug to a predetermined voltage. Specifically, one end of the secondary coil of the spark plug is provided with a central electrode of the spark plug and a constant-voltage element that allows a current to pass therethrough when a voltage across terminals becomes equal to or higher than the predetermined voltage. Another end of the constant-voltage element is grounded.
According to this configuration, when a voltage applied across the electrodes of the spark plug is about to exceed the predetermined voltage, the applied voltage is restricted to the predetermined voltage and flattened. Thus, the conditions of the gas in the gap are made suitable for a discharge to occur for a duration that the applied voltage is maintained at the predetermined voltage, thereby discharge sparks occur in between the electrodes. With this configuration, the discharge voltage of the spark plug is prevented from becoming excessively high and thus the reliability of the spark plug can be maintained.
Owing to the use of the above mentioned technique, the discharge voltage of the spark plug is prevented from becoming excessively high. However, according to the inventors' experiments, it has been proved that the inductive voltage generated in the secondary coil is lowered more than expected. This means that discharge sparks would no longer be generated in the gap of the spark plug and an accidental fire in the engine may occur.
In light of the conditions set forth above, it is desired to provide an ignition system which is able to suppress lowering of inductive voltage generated in a secondary coil and effectively prevent the occurrence of an accidental fire in an engine.